New Grendel upper won't chamber a round properly

[A5BLASTER]

First thing to do is take the extractor off and if it has the rubber oring, get rid of that rubber oring.

Next thing I would check is to see if your hitting the lands, dont trust book data, it's very possible you could have a tight throat and your hitting the lands, if that checks out as good, then move on to sizing the cases another 3 thousands down more and see if that fix's the issue.